The recognition of the adverse effects of inhaled particles especially particles in the sub 10 micron range has led to the development of filter to remove these particles from air streams. These filters are commonly designated as HEPA (High Efficiency Particulate Air) and ULPA (Ultra Efficiency Particulate Air) filters.
Although the acronym, HEPA was not used until the late 1940's, filter media that could satisfy the minimum HEPA high performance characteristics were produced in the late 1930's. A filter material containing crocidolite asbestos in a coarse fiber matrix provided adequate performance with a single layer of media. This media was improved in the early 1940's by the National Defense Research Committee (NDRC) to provide several magnitude of order improvement in performance. Enhanced evaluation techniques also contributed to the development of more effective filters. During the same period, many alternate fine fibers were tested for use in fine particle filters including other types of asbestos, glass wool, rock wool and some organic fibers. HEPA grade filter media was manufactured from all of these filters. However, a media formed of a matrix of asbestos fibers combined with paper fibers was found to be the best for rapid, large scale commercial production. This material was the first to be designated a HEPA media.
After World War II, this industry grew at a dramatic pace driven by the nuclear arms race. The possibility of mutual assured destruction created the need to filter radioactive particles from air breathed by personnel as well as the requirement to remove radioactive particles from the processing environment. Efforts were concentrated on improving performance of the asbestos-matrix fiber media as well as searching for domestic substitutes for the previously imported fibers. A number of natural grasses and regenerated cellulose viscose fibers were found to be suitable substitutes for the matrix fibers. However, only asbestos fibers were utilized as the filter media. In the later 1940's, glass microfibers were developed. They were only available in small quantities for research. The Naval Research Laboratory (NRL) explored the use of glass microfibers in the development of a 100 percent inorganic paper. Pilot production of glass paper began at the National Bureau of Standards (NBS). Full scale production of glass paper began in 1952.
Shortly after the development of microfiber glass paper, a fire occurred in a filtration system at an Atomic Energy Commission (now DOE) using asbestos fiber media containing combustible matrix fibers. The fire was extremely hazardous in widely dispersing the asbestos fibers as an aerosol throughout the facility. It was later discovered that inhaled asbestos fibers were a latent carcinogen. This caused the industry to substitute glass microfibers in HEPA filter media.
Over the next two decades (1955-1975), there were only modest improvements in HEPA media and essentially all of these were in improved performance of the media. This lull resulted from satisfaction with existing media, existence of a substantial nuclear weapon stockpile, and a diversion of funding and research activity to higher priority projects.
However, from 1975 forward, the fast growth of industries such as integrated circuits, microelectronics, and biological industries requiring clean room manufacturing facilities created the need for new media having several orders of magnitude improved performance.
The most common commercial Intermediate Efficiency HEPA and ULPA (Ultra Low Penetration Absolute) media grades sold worldwide are composed of borosiliate glass microfibers bound together by an acrylic resin. However, as recently as 1994, the ASHRAE (American Society of Heating Refrigeration and Air-Conditioning Engineers) industry stopped using glass media and converted to the use of organic fiber media due to concerns with glass fibers dislodging from the media, becoming airbound and inhaled.
The most commonly used organic media is formed from melt blown media and particularly melt blown media that carries an electrostatic charge. Several weights of media are available from several suppliers. Weights range from 10 to 80 grams per square yard. Randomly charged melt blown electrostatic media were tested in accordance with the invention. The pressure drop and efficiencies were not satisfactory to be used to replace the following commercial filters.
TABLE 1 ______________________________________ Type Capture Particle/Micron ______________________________________ Intermediate at least 95% 0.30 Efficiency HEPA at least 99.97% 0.30 ULPA at least 99.999% 0.12 ______________________________________
The medical industry has also adopted electrostatic media for use in filters for spirometry, anesthetic gas, pulmonary function, CPAP sleep apnea apparatus, incubation, respiratory care, breathing circuits and ventilation. The electrostatic media used for these applications is a fibrous, thermo plastic organic media. This media is supplied in grades from 15 grams per square yard to 300 grams per square yard. It is backed with a Nylon or Polyester scrim of from about 0.1 to about 1.0 ounces per square yard on at least one side, preferably both sides and sealed along all its outer edges.
The electrostatic fiber material is formed into a needle-punched, non-woven fabric capable of 99.9% efficiency in the removal of particles from 0.1 to 0.5 microns. Preferably the fibers are organic, thermoplastic mixture of a polyalkylene fiber such as polypropylene and an anionically substituted acrylic fiber. The needle-punched material is very open and some of the fibers are loose and can be dislodged from the fabric. In the manufacture of the fabric for medical applications the fibers are needled together to a layer of scrim material on the lower side. A top layer of scrim is then applied to the needled fabric to assure retention of loose fibers. The top layer seals the edges of the assembly. The efficiencies of these filters need improvement and the edge sealing operation is inefficient and increases the cost of manufacture of the media.